The Third Generation Partnership Project (3GPP) is standardizing the long-term evolution (LTE, also known at evolved universal terrestrial radio access network E-UTRAN or 3.9 G) of the radio-access technology which aims to achieve reduced latency, higher user data rates, improved system capacity and coverage, and reduced cost for the operator. LTE base stations (e-NodeBs or eNBs for short) have a varying number of transmit antennas. Such eNBs employ several semi-statically configured MIMO modes in the downlink. One such semi-statically configured MIMO mode uses power control & control signaling for downlink multi-user multiple input-multiple output (MU-MIMO) transmissions, but details of how downlink DL MU-MIMO will be implemented in LTE are not yet finalized.
The MU-MIMO transmission concept allows transmissions to several users within a cell to be multiplexed spatially at a given time, i.e. these users share the same time/frequency DL resources. Proper user specific precoding at the eNB ensures an acceptable level of intra-cell inter-user interference. The purpose of such MU-MIMO transmissions is to increase the overall cell throughput (i.e. DL capacity) in scenarios with high system load. For example, in the case of MU-MIMO operation with two transmit (TX) antennas at the eNB, the DL transmissions can either spatially multiplex two users in MU-MIMO mode or transmit to a single user with rank 1 transmission. In the case of four available TX antennas at the eNB, there are the possibilities of spatially multiplexing one, two, three or four different users. Generally the different users are referred to as user equipments UEs.
Power control & its signaling to the MU-MIMO UEs plays a crucial role in order to fully achieve the system capacity gains of MU-MIMO. In LTE, it is anticipated that the number of spatially multiplexed UEs may vary from one sub-frame to another, and so the power levels need to be signaled to the MU-MIMO UEs on a per sub-frame basis as part of the downlink control indication (DCI) format on the downlink physical control channel (physical downlink control channel PDCCH in LTE). One current assumption in discussions of LTE MU-MIMO multiplexing has been that MU-MIMO UEs that are spatially multiplexed together all occupy exactly the same physical resources (in time and frequency, i.e., same sub-frame index and physical resource block PRB allocation respectively). In this situation, it is commonly understood that the total available power at the eNB is to be shared equally among the MU-MIMO UEs that are paired (i.e. spatially multiplexed) together.
Consider some examples under the above assumption. If a UE is scheduled alone on given time-frequency resources, the eNB will fully use the available transmit power on these resources to transmit to this UE. If instead there are two, three or four spatially multiplexed UEs on these resources, the eNB will share the available power between the spatially multiplexed UEs, and hence transmit to each of these UEs with ½, ⅓ and ¼ of the total available transmit power, respectively. FIG. 1A illustrates both the assumption and the resulting transmit power for the case of two multiplexed UEs. There is exact overlap of all four of the time and frequency resources (columns of FIG. 1A) between UE1 and UE2, and so when multiplexing its transmissions to these UEs, the eNB will send the transmissions for UE1 with ½ of the total available power and similarly send the transmissions for UE2 with the remaining ½ of the total available power. This follows naturally so that the eNB uses all 100% of its available transmit power over these time-frequency resources. There will be one bit necessary for control signaling of the power to the UEs in this scenario, to indicate to them whether they are to receive at ½ power or at full power (for the case where the eNB transmission is only for one of the UEs, in which case that transmission is with rank 1). FIG. 1B illustrates similarly for four multiplexed UEs. The eNB will send the transmissions for each of UE1, UE2, UE3 and UE4 with ¼ of the total available power to use all 100% of its available transmit power on the multiplexed transmission. In FIG. 1B there will be two bits necessary for control signaling of the power to the UEs, to indicate to them whether they are to receive at full, ½ or ¼ power (the remaining two-bit sequence that is available may be used to indicate ⅓ power for the case of three spatially-multiplexed UEs). This is because an individual UE generally will not know how many other UEs (if any) with which its DL traffic is being multiplexed.
The above considerations lie behind a 2-bit power offset signaling proposal in document R1-082028 (3GPP TSG-RAN WG1 #53, Kansas City, USA, May 5-9, 2008) by Ericsson, entitled “Remaining Details on Control Signaling for the MU-MIMO Transmission Mode” [attached to the priority document as Exhibit A].
This two-bit signaling for power control provides a maximum of four power levels (and one-bit control signaling provides a maximum of two power levels) as noted in the examples above. Consider the number of users being spatially multiplexed to be the integer number N. Then a general statement of the above examples is that there will be N power levels to choose from, the selected power level will be 1/N for any downlink transmission that spatially multiplexes only those N users, and the selected power level will be signaled using one bit for the case of N<3 and two bits for the case of N>2. To the inventors' knowledge, no different proposal for this power control signaling for downlink control information (DCI) format 2 (or any other DCI format) over the physical downlink control channel (PDCCH) has been made. But as will be detailed below, the above assumption leads to sub-optimal results. Limiting the power level options to two or four may be optimum for the case where the resources exactly overlap, but in practice this will typically not be the case; there will be some overlap but it generally will not be a total overlap for all of the N users. For this practical scenario, spatially multiplexing users who have only partial overlap of resources in frequency according to the proposal of document R1-082028 is sub-optimal and leads to inefficient use of transmit power at the eNB in that less than all of its available transmit power is used effectively. These teachings address that inefficient use.